1. Field of the Invention
The present invention relates to a receiver in an OFDM or OFDMA communication system.
2. Description of the Related Art
OFDM (Orthogonal Frequency Division Multiplexing) and OFDMA (Orthogonal Frequency Division Multiple Access) communication systems are widely used in digital terrestrial broadcasting, WiMAX, and other systems. In an OFDM or OFDMA communication system, a transmitter uses transmission data to modulate a plurality of subcarriers the frequencies of which are orthogonally related, the OFDM frequency-domain signals are subjected to IFFT and converted into OFDM time-domain signals, are up-converted into high-frequency signals, and are transmitted through space. On the other hand, a receiver which receives this performs down-conversion of the received high-frequency signals, performs FFT to convert the OFDM time-domain signals into OFDM frequency-domain signals, performs demodulation of the plurality of subcarriers, and extracts the reception data. In such an OFDM or OFDMA system, multiple carriers are used for transmission and reception.
Further, in OFDM or OFDMA, in order to address multipath fading, a portion of the signal at the end of a symbol is copied to the beginning of the symbol to provide a guard interval (GI). And, even when, due to multipath fading, a delayed wave delayed from the main wave or an advanced wave advanced from the main wave is received simultaneously, if the delay amount of the delayed wave and the advanced wave are within the GI interval, there is no interference from the advanced wave or the delayed wave.
However, if a delayed wave and advanced wave occur with a delay amount exceeding the GI interval, adjacent symbols enter within the FFT window, and there is interference from adjacent symbols. That is, intersymbol interference (ISI) occurs, and degradation of the reception signal may occur. Hence there have been various proposals to eliminate such interference waves.
For example, S. Hara, M. Budsabathon and Y. Hara, “A pre-FFT OFDM adaptive antenna array with eigenvector combining”, IEEE International Conference on Communications 2004, vol. 4, pp. 2412-2416, June 2004; T. Uraguchi, N. Kikuma and N. Inagaki, “Application of DCMP adaptive array to OFDM-CDMA communication system”, IEICE Transactions, B, Communications, 83-B(2), pp. 216-224, February 2000; Japanese Patent Application Laid-open No. 2007-6067; and Japanese Patent Application Laid-open No. 2007-6264.
The effect of spatial diversity in suppressing interference waves has the characteristic of depending on the arrival angle for each path; when the arrival angles of an interference wave and the desired wave are close, there is the problem that interference wave elimination is difficult. Also, there is the problem that, when eliminating an interference wave by spatial diversity combining, if the number of paths exceeds the degrees of freedom of the array antenna (the sum of the number of directional beams and the number of directional nulls), interference wave elimination is not possible.